As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries which exhibit high energy density and voltage, long lifespan and low self-discharge rate are commercially available and widely used.
In existing lithium secondary batteries, a lithium cobalt composite oxide having a layered structure is generally used in a positive electrode, and a graphite-based material is generally used in a negative electrode. However, cobalt, as a constituent of the lithium cobalt composite oxide, is very expensive, and use of the lithium cobalt composite oxide in electric vehicles is not suitable due to unsatisfied safety thereof. In addition, the lithium cobalt composite oxide has difficulties in exhibiting high capacity according to energy density increase.
Accordingly, as a positive electrode active material, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure and the like, and lithium-containing nickel oxides such as LiNiO2 is also under consideration. In addition, as a negative electrode active material, a carbon-based material is mainly used, and, recently, a mixture of SiO having 10 times or higher effective capacity than that of the carbon-based material is under consideration due to demand increase for high-capacity secondary batteries.
However, lithium secondary batteries have various problems. For example, some lithium secondary batteries have problems related with characteristics in manufacturing and operating a negative electrode.
For example, in an initial charge and discharge process (activation process) of a carbon-based negative electrode active material, a solid electrolyte interface (SEI) layer is formed on a surface of the negative electrode active material and, accordingly, initial irreversibility is induced. In addition, an SEI layer is collapsed in a continuous charge and discharge process and an electrolyte solution is depleted in a regeneration process, whereby a battery capacity is reduced.
Furthermore, when a carbon-based material and SiO are mixed, problems according to SEI layer formation can worsen with increasing cycle number because SiO causes greater electrode expansion, compared to the carbon-based material.
So as to address such problems, it was attempted to form an SEI layer having far stronger binding force, to form an oxide layer or the like on a surface of a negative electrode active material, or the like. However, such an SEI layer does not exhibit commercially applicable characteristics due to electrical conductivity decrease by the oxide layer, productivity decrease by an additional process, etc.
In addition, an additive may be added to an electrolyte solution. However, a main object of an electrolyte solution additive conventionally used is to prevent side products generated during charge and discharge.
Therefore, there is an urgent need for technology to fundamentally resolve such problems.